The goal of the work described in this thesis was to investigate the mechanism of embryonic head development in Drosophila melanogaster. I focused on the regulation of the cephalic gap gene orthodenticle (otd), which specifies anterior head structures in the Drosophila embryo. otd has been proposed to be a direct target of the morphogen Bicoid (Bcd), which is required for specifying both head and thorax. It had also previously been proposed that the levels of Bcd expression are responsible for this differential specification. These hypotheses however have remained untested. In this thesis, I investigated the genetic and molecular requirements underlying the regulation of otd expression in the embryonic head. In the first part of this study, I determined the genetic requirements for correct otd expression. I examined mutations known to affect the development of the head or adjacent regions to test their effect on otd expression. Contrary to early models of embryonic pattern formation, I found that high Bcd levels are required neither for otd activation nor for the establishment of cephalic structures. Instead, I showed that the terminal genetic system contributes positively to otd activation in the head. This contribution is critical when Bcd levels are low. In addition, I identified a novel pathway mediated by the terminal gap gene huckebein (hkb) through which three maternal systems cooperate to repress otd expression at the anterior terminus of the embryo. In the second part of this study, I investigated the molecular mechanism of otd regulation. In a systematic deletion analysis of the otd regulatory region, I identified two independent otd enhancer elements, 186 bp and 173 bp in size respectively. Each of these elements is sufficient to drive early otd expression in the head. I then showed that otd is a direct target of Bcd. This study is the first demonstration that a cephalic gap gene is directly regulated by the Bcd morphogen. I also studied the role of Bcd binding sites in targeting expression of a cephalic gap gene to the head and found that the affinity as well as the number of Bcd binding sites do not determine the spatial extent of otd expression, but only its intensity. Surprisingly, I also found that early zygotic gene expression can be targeted to the head primordium without direct Bcd regulation and that normal otd head expression requires repression of otd at the posterior terminus of the embryo. In summary, this work shows that otd is directly activated by Bcd. However, Bcd is not the only factor which restricts otd expression to the head. Instead, other regulators contribute to the cephalic localization of otd. The direct and indirect Bcd response elements, the terminal system response elements, as well as complex interactions among these elements are all necessary for correct regulation of otd expression. This study demonstrates that directing gene expression to the anterior region of a developing animal embryo requires the cooperation of multiple genetic systems.
Thesis (Ph.D. in Biology) -- University of Pennsylvania, 1998. Source: Dissertation Abstracts International, Volume: 59-04, Section: B, page: 1519. Adviser: Robert Finketslein.